The present invention relates to a method of controlling the braking effect during engine braking in an internal combustion engine having an exhaust turbocharger unit with adjustable guide blades for controlling the turbine speed and gas flow.
In an internal combustion engine having a turbocharger unit with variable turbine geometry (VTG), it is possible to control the gas flow through the turbine. For example, control may be achieved by means of rotatable guide blades upstream of the turbine rotor.
Alternatively a so-called “sliding wall” may be used in order to vary the intake area of the turbine.
In order to control the engine braking effect of a diesel engine which is equipped with VTG and a compression brake, which can be charged by pressure charging of the engine cylinders from the exhaust system, and/or by pressure charging via the intake side of the engine, the turbine guide blades are set to a certain position which will give a certain braking effect. When the braking effect is to be varied, the guide blades are moved, the braking effect being modified since the charging pressure (intake side) and counterpressure in the manifold are adjusted.
In engine braking tests, for example by simulating downhill driving with a heavy vehicle having an engine equipped with VTG, it has been shown that the braking effect is not kept constant when the brake is engaged for some time. This is due to the fact that temperature changes affect the position of the guide blades since guide arms and similar arrangements alter their length. Particular problems may arise, for example, if the guide blades are in a position in which the turbocharger is virtually shut-off. In the aforementioned tests a slight change in the guide blade position was capable of exerting a relatively strong influence on the braking effect. It proved impossible to set the desired braking effect by moving the guide blades to a preselected position. It was partly a problem of longitudinal thermal expansion and partly a problem of the preselected mechanical position varying somewhat from time-to-time due to tolerances.
These problems can mean that the control system becomes slow and unstable with pressure fluctuations on the intake side of the internal combustion engine. Owing to these unwanted pressure fluctuations, problems arise in controlling the braking effect. There is also a risk of reduced comfort and problems of synchronization with other service brakes, such as a retarder, in a vehicle, the brake system of which is controlled by means of the vehicle's various control units. In the worst case it is also conceivable that the braking effect might become too great for components of the drivetrain, if these components are operating close to their strength limit.
When using a turbocharger with variable turbine geometry to control the engine braking effect, it is important to have control of the guide blade position, the charging pressure and possibly also the inlet pressure and the turbine speed, so as to be able to provide a uniform and consistent engine braking action. One method of controlling the guide blades in a turbocharger having variable turbine geometry is disclosed in U.S. Pat. No. 6,085,526. This describes a solution in which the charging pressure and manifold pressure are combined in order to control the braking effect. Further efforts are needed, however, in order to remedy the problems described above. For example, variations in the engine valve lift can affect the manifold pressure which means that the control strategy proposed in the patent cited will not work.